Patients must complete a questionnaire prior to CT in order to assess their suitability for CT and IV contrast. IV contrast contains iodine, so patients with an iodine allergy, an allergy to shellfish or a previous history of reaction to IV contrast should have a non‐IV‐contrast CT.

Emergency equipment must be available if IV contrast or a general anaesthetic is used (Chernecky and Berger [32]).

The patient will be cannulated for the intravenous injection of iodine‐based contrast medium to enhance the blood vessels and bodily organs. During the injection, it is normal for the patient to transiently feel warm over their whole body and to experience a metallic taste at the back of the throat, and some patients feel nauseous. Some CT examinations, such as CT pulmonary angiograms, demand contrast to flow at a fast rate and therefore a large‐gauge cannula (20 G) is required. CT‐compatible ports and peripherally inserted central cannulas (PICCs) may also be used (Thomsen et al. [240]). Antihistamines and steroids may be required to treat a reaction (Pagana and Pagana [177]).

Although CT is a fast imaging technique (5–20 seconds), some patients are very concerned about feeling claustrophobic during scanning. However, with kind, careful explanations, most manage to be scanned. For more information, see the advice on claustrophobia above in the section on MRI.

Patients are prepared for CT by drinking an oral contrast, usually water, which shows the digestive tract. This serves to hydrate the patient, which is beneficial post‐IV contrast, and also acts as a negative contrast agent in the digestive tract. Patients must refrain from eating for 2 hours prior to CT in order to allow them to drink easily and to reduce nausea after the IV contrast medium (Keogh [114]; Thomsen et al. [240]). IV access must be established prior to the CT scan to facilitate injection of the IV contrast. Radiopaque objects, such as jewellery, must be removed (Chernecky and Berger [32]).